1. Field of the Invention
The present invention is related to the field of metal casting and, more particularly, to a system and method for producing foundry quality sand from non-conventional starting materials, and for classifying the sand so produced into two or more foundry grade products.
2. Description of the Related Art
Most foundry sand is made by sieving or wet classifying naturally occurring silica or quartz sand. (As used herein, xe2x80x9cquartz sandxe2x80x9d is intended to refer to sand containing silica as is found in quartz in crystalline form. As used herein, xe2x80x9cnon-quartz sandxe2x80x9d is intended to refer to sand which does not contain a significant amount of silica.) Quartz sand suitable for casting contains low levels of compounds of alkali and alkaline earth metals, of both organic and inorganically bonded carbon and of halogen and sulphur derivatives. Such sand consists of rounded particles with weight average mean particle sizes of 0.15 mm or more and narrow size distributions, with typically more than 90% of the particles within 0.5 to 1.5 of the mean.
In some cases, the thermal or physical characteristics of quartz sand are unacceptable and foundries are obliged to use other sands with better properties. These non-quartz alternatives are much less common and greatly more expensive than quartz sand and include olivine (ferriferous magnesium silicate), chromite (ferrous chromite, FeCr2O4), and zircon (zirconium orthosilicate, ZrSiO4). The greater expense of the alternatives to quartz proscribes their general use, and foundries that make particularly demanding precision parts commonly use quartz sand or a recycled sand mixture containing an appreciable fraction of quartz sand for making the external parts of molds, and new non-quartz sand for making the internal parts or cores of the molds.
Foundry sand must resist the temperatures encountered in the casting process, and should not react adversely with the binders used to make molds and cores. It should pack well so that its bulk density is high, yielding a smooth surface on the cast metal product, yet be porous enough to allow the easy escape of gas formed during casting. High bulk density is achieved by using naturally occurring rounded particles that can easily move over one another and which have as broad a size distribution as possible. However, good porosity requires low levels of fine particles, whilst smooth casting surfaces require low levels of large particles; both of these factors limit the breadth of the particle size distribution. A typical high quality quartz sand consists of rounded grains whose particle size distribution is a compromise between these demands, with at least 95% of the particles being within xc2x175% of the mean size and with less than 2% of the particles being below one quarter of the mean size.
The combination of physical and chemical properties required of a quartz foundry sand limit the number of locations where such products occur naturally. Sand may therefore need to be shipped over considerable distances, making quartz foundry sand considerably more expensive than local ordinary builder""s sand. Many countries, particularly those located in the drier parts of the world such as northern Africa and the middle East, lack indigenous sources of quartz suitable for use as foundry sand and must import foundry sand at considerable cost from northern and western Europe.
A further factor limiting the number of locations that can supply quartz foundry sand is that much quartz sand, e.g. beach sand, is contaminated with shell or bone fragments or limestone particles that seriously interfere with casting procedures. Such interference is created by the fact that these contaminants may react with commonly used binders and/or decompose at the temperatures typically used to cast metals.
Not only does quartz present difficulties in availability, the use of quartz has been associated with respiratory ailments. The World Health Organization has officially classified quartz dust as a carcinogen. Hence, quartz sand is the subject of restrictions and precautions in the workplace, and the spent sand, particularly the dust from foundry filters which contains elevated levels of quartz dust, is similarly restricted. This limits the useful employment of spent quartz sand in concrete and asphalt.
Another weakness associated with quartz is its non-linear coefficient of thermal expansion. Quartz undergoes a crystalline transition at ca. 560xc2x0 C. which is accompanied by a considerable increase in volume. Since different parts of the mold are at different temperatures during casting, they expand unevenly and cracks develop, into which molten metal can penetrate. After casting, these metal intrusions appear as thin wafers that protrude from the casting and have to be removed in time consuming finishing operations. At worst, the cast part may need to be scrapped. This phenomenon, known as xe2x80x9cfinningxe2x80x9d is the most common cause of scrap in metal casting.
Like quartz, the currently available alternatives to quartz are also environmentally suspect. Olivine is highly alkaline and can contain nickel and in some cases asbestos, all of which can cause irritation to skin and lungs; together with chromite both are considered toxic waste and must be disposed of in special dump sites. Zircon is weakly radioactive, requiring workplace precautions and dump site limitations.
The sources of currently used alternatives to quartz sand are far fewer in number and most are located outside of the areas where there are large numbers of foundries; this means that they bear considerable freight cost penalties compared to quartz sand. Furthermore, and unlike quartz sand, they also have relatively highly valued alternative applications. For example, zircon and olivine are used in the manufacture of refractories, whilst chromite is the ore used in the manufacture of chromium metal. These factors make these alternative sands as much as ten or twenty times more expensive than quartz sand and they are therefore rarely used as the sole sand in a foundry.
Given the difficulties in obtaining suitable sand, it is important to consider the xe2x80x9clifexe2x80x9d of the sand. After use, foundry sand is either dumped, used for non-foundry purposes such as construction materials or reused. Because spent foundry sand can contain organic materials, acids and heavy metals, environmental authorities usually insist that it must be dumped at an approved site for toxic waste; this adds considerably to the foundry""s total sand related costs. Financial and environmental considerations encourage measures that minimize the net use of sand, including recovery and reuse of the sand by recycling the spent molds and/or cores. For these reasons, many foundries find it economically viable to install equipment that recovers and reuses spent sand.
The reuse of spent sand requires that extraneous material such as char and residual binder be removed as completely as possible. Spent molds and/or cores are broken into smaller and more easily handled aggregates, typically using a vibrating screen. Char and residual binder are then removed. Sand recovery equipment typically uses either thermal or mechanical methods.
Thermal treatment entails heating the sand to 700xc2x0 C. or more in an excess of air so that organic binders are burnt off. The treated sand is then fluidized in an air stream to remove dust before being reused. Such thermal processes remove organic binder residues by incineration; they yield sand of fair quality but are energy intensive, costly and not suitable for all sand/binder combinations. They also lead to emissions of environmentally undesirable gases (oxides of sulphur, nitrogen and carbon).
State of the art attrition involves gently and repeatedly rubbing the sand grains against one another so that loosely held interstitial binder and char is converted to dust. Such mechanical processes are less costly but the quality of the recovered sand is inferior and its use within the foundry often more restricted than that of new or thermally reclaimed sands. Both thermal and mechanical recovery methods remove dust by means of cyclones or fluidized beds.
Recovery of used sand is significantly complicated by the fact that different sand types are sometimes used for the molds and cores. Once the casting process is complete, it is rarely feasible to separate the used molds and cores from one another, so the different sands used for these two purposes become mixed. State of the art recycling methods are unable to satisfactorily separate this mixture into its component parts and foundries that use both costly non-quartz sand and cheaper quartz sand must therefore replenish their non-quartz sand with new material after each casting cycle.
In other cases, foundries that would prefer to use and recycle two grades of the same sand, e.g., one for making the mold and another of different particle size distribution for making the core, are unable to do so because limitations in state of the art recycling methods do not allow such closely similar materials to be easily separated. They must therefore either choose to compromise by selecting and recycling one grade of sand for all purposes, or continually buy new sand for the one application and use a suboptimal mixed recycled product for the other.
The proportion of sand that can be recycled can also be limited by the binder system used, since some binders react with quartz at casting temperatures; these include some of the most commonly used binders that contain highly alkaline materials such as sodium silicate or mixtures of phenolic resins with caustic alkalis. These binder resins are difficult to remove, either by attrition or thermal treatment and, when heated during thermal recycle or subsequent casting, may react with the sand to form silicates of low melting point that seriously compromise the refractory characteristics of the sand.
Foundries are also limited in their choice of classification methods for sand recycling and cannot economically employ methods originally used in large scale manufacture of foundry sand. Wet classification has inordinately high operating costs and yields effluents that pose environmental hazards. Sieves are difficult and costly to use with fine materials and, unless the product fractions are carefully remixed, fail to yield products whose particle size distributions give optimal packing characteristics.
In view of the foregoing, one object of the present invention is to overcome the difficulties of procuring suitable quality foundry sand through a system and method of producing foundry sand from alternative materials and providing for the recycle of such sand.
Another object of the invention is to achieve close control of both particle shape and particle size through the combination of a mechanical oolitization procedure followed by air classification.
A further object of the invention is a system and method that enables use of locally available, less expensive, quartz and non-quartz materials previously considered unsuitable for foundry sand.
Yet another object of the invention is a system and method for recycling molds and cores to separate and reclaim the sand contained therein for reuse.
An additional object of the invention is a particle classification system that allows for the simultaneous recovery of two or more distinct grades of foundry quality sand from a single input stream.
In accordance with this and other objects, the present invention is directed to the combination of a controlled energy particle-on-particle attrition unit followed by a multi-fraction classifier. Incoming particulate material, which may constitute either raw material for and/or used sand from cores and molds, is placed within the controlled energy attrition unit where the particles collide with one another. Through these collisions, edges, surface projections and coatings of the particles are chipped away but the particles themselves are not crushed. This oolitization procedure rounds and cleans the particles, yielding a sand stream having particles covering a wider size distribution. The sand stream is then directed through the multi-fraction classifier where the sand is classified into two or more useable grades of foundry sand.
These and other objects of the invention, as well as many of the intended advantages thereof, will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.